Sound-transmission, sound-recording, and sound-reproducing system



Nov. 24,1936. 4, D, B L N 2,062,275

SOUND TRANSMISSION, SOUND RECORDING, AND SOUND NEPRODUOING SYSTEM FiledNov. 8, 1934 2 sheets-sheet 1 Nov. 24, 1936. A. D. BLUMLEIN 2,062,275

SOUND TRANSMISSION, SOUND RECORDING, AND SOUND REPRODUCING SYSTEM FiledNov. 8, 1934 2 Sheets-Sheet 2 may assume, but it must be understoodthat.

Patented Nov. 24 1936 UNITED STATES PATENT orr cs 2,062,275SOUND-TRANSMISSION,

ING, TEM

SOUND-RECORD- AND SOUND-BEPRODUCING SYS- Alan Dower Blumlein, WestEaling, London, England, assignor to Electric 82 Musical IndustriesLimited, Hayes, England Application November 8, 1934, Serial No. 152,147In Great Britain February 10, 1934 6 Claims. (01. 179-1) This inventionrelates to systems for the electrical transmission of sound, andcomprises an improvement in or modification of the invention disclosedin the specification of application No. 647,057 filed the 13th day ofDecember,

1932. In that earlier specification are shown systems in which soundsare picked up by a plurality of microphones for transmission 'in such amanner that when reproduced the sounds convey a binaural impression to alistener. In the The arrangement may consist in the combination of avelocity microphone and a pressure mi-' Volume V, page 139 et seq.,October, 1933, and

diiierent forms of the present invention may be variously regarded as amodified use of such a device in a system such as described inapplication No. 647,057 filed the 13th day of December,

1932, or as a system such as previously described in application No.647,057 filed the 13th day of December, 1932, embodying one or moremicrophones of that character in place of the microphones originallyindicated. Alternatively there may be employed in accordance'with thepresentinvention other forms of microphones embodying combinedvelocity-responsive and pressure-sensitive elements, different from thatdescribed in the above-mentioned publication.

The invention will be readily understood from the following descriptionof a few forms which it these forms are described merely by way ofiilustration and not of limitation to the scope of the invention.Understanding of thedescription will be facilitated by reference to .theaccompanying drawings representing arrangements in accordance with theinvention.

In these drawingsz- Figures 1 and '2 represent two forms ofthe inventionin symbolic fashion:

Figures 3 to 6 represent diagrammatically one form of an arrangementpreviously described in the specification of U. S. A. patent applicationNo. 647,057 of 13th December, 1932, and

Figures 7 and 8 represent front and side elevations respectively of aform of known compound microphone which may be used in a novel manner inconnection with the present invention;

In carrying the invention into eifect in one convenient manner a soundtransmission system is arranged whereby sound impulses picked up by aplurality of microphones are transmitted over suitable channels andreproduced in a plurality of loud speakers, substantially .as describedin application No. 647,057, filed the 13th day of December, 1932. Thesystem may incorporate recording and reproducing stages, andthetransmission is not necessarily over telephone or like lines but may,for example, be eifected by duplex wireless telephony.

To facilitate understanding of the present specification a briefdescription of the invention described in the above mentioned priorspecification will be given. One arrangement of that invention is shownin Figures 3 to 6 herein. .Sounds from a source s are picked up by twopressure microphones a1, a2, amplified if necessary by amplifiers111,172, passed through interacting networks 01 (described more fullybelow), the outputs again amplified if necessary by amplifiers d1, d2,and 'fed to loud speakers e1, e2. The interacting networks 01 includeinitially an arrangement of transformer windings shown in Figure 4 soarranged that their outputs comprise respectively half the sum and halfthe difference of the inputs. be represented diagrammatically by thesymbol shown in' Figure 5. Figure 6 represents the complete networkarrangement 01 of the. circuit shown in Figure 3, and it will be seenthat after passing through transformer windings .a: the impulses passthrough networks comprising, in one channel the resistances 94, I14 andi4, and in the other channel resistances d4, erand I04, and condenserii. The modified impulses are again passed through transformer windings3 (whereby half their. sum and half their-difference: is againobtained), and fed to the amplifiers (11, (In and loud speakers e1, 62.'It is shown in the prior specfication referred to that by suitablechoice and arrangementof the elements in this circuit a Such anarrangement may directional sound effect is conveyed to a listener bythe loud speakers an e: depending upon the d!- mitting microphones a1,a: may be replaced by a velocity microphone, such as a moving stripmicrophone, in close .iuxtaposition to a pressure microphone, such as acarbon or crystal microphone. While the invention is not limited tothese two forms of microphone (i. e. moving strip and carbon orcrystal)they are mentioned in view of the fact that they can be made of verysmall dimensions which allow them to be brought close) together. Thesetwo microphones are connected together so that their outputs 'are fed totwo channels leading to later stages of the system, and the arrangementis such that one of these channels receives and transmits the sum of themicrophone outputs while the other re-.

ceives their difference. Any convenient arrangement of electricalelements may be used to provide this sum-and-dlflerence step, andseveral arrangements will be found described in the abovementionedspecification and one such arrangement is shown in Figs. 4 and 5 anddescribed above.

The outputs so obtained after this summing and differencing processconsist of currents which fora particular sound source are in phase butwhose relative magnitudes depend on the direction of the sound wavearriving at the microphone, and these can be made suitable for obtainingthe desired binaural effect by arranging the velocity microphone to beinsensitive to sound arriving from the central" direction, so thatoutputs, are obtained from microphones which are equivalent to theoutputs of the first sum" and difference channels (Fig. 6 herewith).

For example, with such an arrangement the output of the pressuremicrophone (after amplification orattenuation) is comparable with theoutput obtained from the resistance In shown in Fig. 6 herewith(corresponding to Figure 5 of application No. 647,057, filed the 13thday of December, 1932) while the output from the velocity microphone iscomparable with the output obtained from the resistance e4 of thatfigure, and by providing theabove-mentioned sum-and-diference processonce only to these microphone im-- pulses, therefore, outputs areprovided suitable for feeding to the loud speakers.

The transformer arrangement a: and the modii'ying networks comprisingelements (14, 64, f4. 94,

71.4, 1'4 and k4 shown infFigure 6 can'thereiore be eliminated and-amuch simpler arrangement is available giving the same effect.

This will be seen clearly by referring to Figure 1 wherein 4 representsa directionally sensitive velocity microphone, e. g. a strip microphone,while b represents a pressure microphone, which is not directionallysensitive. These are connected to a network represented at c by thesymbol introduced in Fig. 5 herewith (corresponding to Figure 4 ofapplication No. 647,057 filed the 13th day of December, 1932), and theoutputs pass through. leads d and e to two loud spreakers.

The strip of microphone a is arranged along the axis of the sound fieldand is clearly insensitive to sounds from a sound source 81 on thataxis, but has maximum sensitivity tosounds from a lateral source 8:,whereas the sensitivity of microphone b is substantially independent orthe position of the source. Leads (1 and e transmit respectively halfthe sum and half the diflerence oi the microphone outputs, and clearlywith the rection oi the sound source s. relatively to the misound sourcein position 81 the two outputs in leads d and e are the same (so thatloud speakers connected thereto would be equally loud and indicatecorrectly a central position of the sound source) whereas with the soundsource in position S: the microphone outputs are the same, and theimpulses in leads d and e are therefore respectively a maximum value andzero (which provides full sound in one loud speaker and sube stantiallynone in the other, thus giving the correct impression of a laterallydisplaced sound source).

An alternative arrangement of the present in-- vention which may be used(especially where th microphones are of similar impedance, as in th caseof microphones having two strip elements described more fully below) isshown in Figure and consists in connecting one terminal of on microphoneto one of the other microphones and connecting the other microphoneterminals respectively to opposite ends of the primary winding 1 of atransformer. The primary 9' of a second transformer is connected betweenthe common microphone terminals and the mid-point of the first-mentionedtransformer primary fmand it will be clear that while the output fromthe secondary of one transformer will consist in the sum of themicrophone impulses that from the other will consist in theirdifference. By suitably arranging the orientation of the microphones(one of which is, of course, directionally sensitive), and by suitablychoosing the transformer ratios in order to obtain the desired ratio ofoutputs in the two channels connected to the secondary windings of thetransformers, the impulses can be controlled satisfactorily to providethe binaural eiIect aimed at by the system, and the relative amplitudesof impulses in the two channels determined in accordance with the.dlrection from which the sounds reach the microphones.

Instead of employing two separate microphones, the microphone elementsmay be arranged together to form a single instrument. An instrument ofsuch a character has been described in the Journal of the AcousticalSociety of America, Volume V, page 139 et seq., October, 1933, and alsothe specification of British Patent No. 405,497, and as shown in Figs. 7and 8, comprises a strip a: supported from its ends between poles of afield magnet in and having midway along its length a supporting bridgeca which efiec-' tively divides the strip into two separately opertiveportions, one of which is allowed to function as an ordinary velocitysensitive element, while the other is loaded on one side by an acousticimpedance (consisting of an adjacent tube (is packed with feltabsorption pads) whereby it is rendered sensitive to pressure variationsand substantially non-directional. Such a microphone may readily beemployed in the present invention, the elements being arranged andelectrically connected in the manner described above. Alternatively themicrophone may be slightly modified to meet the requirements of thisinvention. For example instead of being formed of one strip two separateelements may be located between the magnetic pole pieces-side by sideand substantially parallel to one another, one being acoustically dampedas indicated above, this arrangement allowing relative angulardisplacement of the planes of the elements, should that be necessary ordesirable to achieve the desired employed in respect of only part of thefrequency range being transmitted. For example in copending applicationNo. 750,898, filed October 31, 1934, it is shown that in somecircumstances it isdesirable initially to transmit low frequencies overone pair of channels and high frequencies over another pair of channels,and the systems described in this specification may if desired beemployed for one part of the range, in combination with a differentsystem or systems for another part or parts of the range.

Finally it must be understood that the invention is not limited solelyto the details of construction or arrangements described herein sincemodifications may be introduced as they become desirable or necessary inorder to carry the invention into effect under different conditions andrequirements which have to be fulfilled, without departing in any wayfrom the scope of the invention.

Having now described my invention, what I claim as new and desire tosecure by Letters Pattent is:- v

1. A system of sound transmission comprising a plurality of loudspeakers, a velocity microphone element and a pressure microphoneelement, a plurality of channels connecting said microphone elements tosaid loud speakers, and means by which the impulse transmitted in onechannel is composed of half the sum of the microphone outputs while theimpulse transmitted in another channel is composed of half thediiference of the microphone outputs.

2. A system of sound transmission according to claim 1 comprisingtransformers in one of which the microphone outputs assist, and inanother of which the microphone outputs oppose, one another.

3. A system of sound transmission comprising a plurality ofloudspeakers, a velocity microphone element, and a pressure microphoneelement, and a plurality of channels, connecting said microphoneelements to said loudspeakers,

so arranged that the output intensities of the loudspeakers differ fromone another by an amount dependent upon the output of the velocityelement and their relative output intensities vary in accordance withthe direction from which the sounds are incident upon the microphoneelements.

4. A system of sound transmission comprising a velocity-sensitivemicrophone element and a pressure-sensitive microphone element arrangedin close juxtaposition, means for causing the output channels of saidmicrophone elements to interact with one another, and a plurality ofloudspeakers connected to receive impulses resulting from saidinteraction of the microphone outputs, the arrangement being such thatthe loudspeaker outputs diifer by an amount proportional to the outputof the velocity microphone, and the relative intensities of theloudspeakers vary in ac- 1 cordai ce with the direction from which thesounds approach the microphones.

5. A system of sound transmission comprising a velocity-sensitivemicrophone element and a pressure-sensitive microphone element arrangedtogether in a single instrument, each element being connected to thetransmission circuit as a separate microphone, a. plurality ofloudspeakers by which impulses from said microphones are reproduced, andmeans adapted automatically to control the relative intensities of saidloudspeaker outputs in accordance with the direction from which thesounds to be transmitted reach said microphone elements.

6. In a system of sound transmission according to claim 5 a microphonecomprising, in a single instrument, a strip element sensitive to thevelocityof the air due to the sounds received, and another strip elementacoustically loaded to render it sensitive to. pressure variations ofthe air due to the sounds received, each element being connected to thetransmission circuit as a separate microphone.

ALAN DOWER Brim/mam.

